Both PBX and a composite propellant are high filled composite materials which are mainly comprised of explosive particles and polymer materials.
Ever since the development of PBX on 1960, as for a binder for a composite propellant and PBX, inert binders such as polyesters, epoxide, polysulfide or polyurethane binders have been majorly used. Currently, hydroxyl-terminated polybutadiene (HTPB) has been reported to be a binder having the most excellent performance.
Although conventional polymer binders are insensitive, they are non-energetic, therefore it has been raised as a problem that the reduction in the whole energy density of the high performance PBX of which demand is increasing. Since PBX is generally prepared by homogeneously mixing solid molecular explosive particles and a binder and casting the mixture, there are many limitations in the preparation of high performance PBX or propellants which use a high energy oxidizing agent due to problems such as compatibility between two components. Further, high performance explosives or high performance propellants have high sensitivity of the final explosive product, being potentially dangerous. In this regard, in order to prepare explosives having low sensitivity as well as high performance, a method to adjust to reduce the degree of filling with solid explosives by using a high energy binder has been recently proposed.
In recent years, with an effort to achieve high performance PBXs and improved performance in composite propellants, many studies regarding a high energy binder and a high energy plasticizer which contain high energy functional groups such as GAP, poly(3-nitratomethyl-3-methyl-ocetane) (hereinafter, poly(NIMMO)) and poly(glycidyl nitrate) (hereinafter, poly(GLYN)) have been made, and examples showing their applications to some high performance PBXs or complex propellants have been reported.
However, high energy polymer binders such as such GAP, poly(NIMMO) and poly(GLYN) which still have been being studied so far potentially have many problems in terms of thermal stability, compatibility with explosives, processability and mechanical properties.
For the application as a novel high energy binder material, requirements in properties such as high energy and density contained in a unit mass, excellent safety (abrasion sensitivity and impact sensitivity, etc.), heat resistance and time-dependent property changes, a safe preparation process and low cost requirements should be satisfied. Among these requirements, safety properties such as abrasion sensitivity or impact sensitivity, etc. are not easily anticipated on a theoretical basis, which have great effects on putting it to practical use. Compatibility with a high energy density oxidizing agent is also one of the considerable requirements for selection for practical use.
The above-explained high energy binders are prepared through a curing reaction between a prepolymer which is in the form of a high energetic polyol and a curing agent corresponding to the prepolymer, i.e., for example polyfunctional isocyanate. Most of currently used high energy polyols consist of polyether-type backbone chains, which are prepared by a cationic ring-opening polymerization of oxirane or oxetane monomers containing energy groups such as —N3 and —ONO2.
Among the conventional high energetic binders, GAP has high density (1.29 g/mol), great heat of formation (+176 kJ/mol) and emits a large amount of energy during the break of —N3 bindings and then generation of N2 therefrom. Based on such properties, after 1972 when its synthesis have been first reported, from 1980 to the present, it has been one of the most studied high energetic polyols for the use of a high energetic binders. GAP binder is a eco-friendly high energetic material with less chlorinated gas, have good compatibility with most high energy oxidizing agents and insensitive properties. Therefore, GAP has been drawing an attention as a low smoke propellant or a low pollutive compound and its synthesis, structure, thermal behavior, physical/chemical properties are studied extensively.
Among GAPs, those having a typical molecular weight of approximately 1700±300 g/mol are used as a binder, which have a low glass transition temperature (Tg=−45° C.) and excellent thermal stability (Td=200-247° C.). However, a high performance PBX and composite propellant prepared by using GAP as a prepolymer in conventional arts, as described above, have problems in mechanical properties owing to degradation in properties such as elasticity by —N3 substitution groups, and particularly they are reported to have a disadvantage of losing flexibility at a low temperature. Based on such reasons, in practical applications as a binder to explosives and propellants, an excessive amount of a plasticizer 3 to 4 times as much as the amount of polyol is used, which causes migration of the plasticizer and debonding between the explosive or propellant and a liner. Further, gas generation resulted from side reactions between impurities and moisture generated during a hardening process is also an additional problem in terms of processability, and in order to overcome such problem, various catalysts for curing reaction are reported to be under research.
As for studies regarding GAP, synthesis of GAP-THF copolymer by Y. M. Mohan and K. M. Raju has been reported, however it has low difunctionality and any improvement in polyurethane properties by using it has not been reported.